STRUCTURAL TRANSITIONS IN FINEDISPERSIONAL MANGANESE PHASES

G.N. Lysiuk

Institute of Geology, Russian Academy of Sciences, 54, Pervomaiskaya st., Syktyvkar, 167610, Russia, cryst@geo.komi.ru

Keywords: structural transition, finedispersional phases, experimental modelling

Structural transformations study of fine grained manganese minerals was conducted in three directions:
a) phases transitions study in native conditions;
b) high temperature heating in dry air;
c) experimental modelling at high temperatures and pressures in different media (neutral, acid, alkaline).

The object of our investigations is fine-grained minerals from pelagic ferromanganese nodules. There are three different crystalline manganese phases in the nodules: 10A manganite, 7A manganite and vernadite. Among the 10A mineral series there are minerals with tunnels and layered crystal structures. Todorokite is a mineral with tunnel structure and asbolan has laerd structure. 7A manganite is called bernessite and has layerd structure too.

Under high temperature heating in dry to 300^oC 10A phase are changed. Its way to transform is dependent on which mineral it represents. At temperature above 550^oC ways to transform are dependent on chemical composition of original matter. Formation of jacobsite is essential for all samples. If nodules are enriched in iron then gematite is formed, if they are enriched in manganese then bixsbyite is formed.

On the basis of hydrothermal experiments filds of crystalline phase stability were revealed. Under hydrothermal conditions weakly crystalline minerals and roentgenamorphous phases transform to phase with stable structures. Products of experimental modelling in neutral media are devided into mineral phases with two structure types: olivine structure type (tephroite, fayalite) and spinel structure type (hausmanite, magnetite, ferrites and manganites of nickel and cobalt).

Under hydrothermal conditions in acid media hematite is formed from roentgenamorphous compounds of iron in the 3⁺ valent state. Moreover initial crystalline phases do not changes at temperatures below 350^oC. And at temperature above 350^oC new crystalline phases (jakobsite, tephroite, braunite) are formed from initial crystalline phases. At 500^oC braunite transform to hausmanite. The formation of gematite and braunite under treatment in acid media is in accordance to results of investigations of mineral phases in metamorphic ferro-manganese nodules. It is important from the point of view ferro-manganese ore genesis since we can now describe processes of recrystallization of manganese and iron hydroxides and the formation of the elements oxides.

Under hydrothermal conditions in alkaline media there formed following associations of mineral phases: asbolan; asbolan-braunite; asbolan-hausmanite; jakobsite-tephroite.